The present invention relates to a continuously variable transmission comprising a rotatable drive and driven pulley, each of which having an axially displaceable sheave for transmission ratio control by varying the running radii of a power transmission belt running between the sheaves; and a lubrication nozzle having an output for supplying a lubricating fluid to a midpoint at the inner side of the belt.
The present invention also relates to a lubrication device suitable for application in a continuously variable transmission (hereinafter also referred to as CVT) and to a vehicle provided with the continuously variable transmission.
It is generally known in the art to increase the performance of a CVT by appropriate cooling and lubrication of the transmission, in particular of the belt. For this purpose it is known to provide an oil supplying tube in the transmission for exhausting pressurised medium in the direction of the belt""s inner side. An improvement is known from EP-A-0 688 980, disclosing a hydraulic supply tube having several discharge openings, shown directed to locations of entry of the belt at each pulley.
Such a continuously variable transmission acknowledged in the preamble of claim 1 is generally known in the art, in particular from WO 98/20269 (EP-A-0866929). The known transmission comprises a rotatable drive or primary pulley and a rotatable driven or secondary pulley, both being rotatable about parallel axes. Each of the pulleys have an axially displaceable sheave or pulley half, and a fixed sheave. By axial displacement of the sheaves concerned running radii of a torque transmission belt running between the sheaves are continuously variable for varying the transmission ratio of the transmission. The transmission further comprises lubrication nozzles having outputs for supplying lubricating oil to midpoints of the belt. In particular these outputs open at two parallel and axially offset hypothetical planes C and Cxe2x80x2 respectively. The first output extends perpendicularly to the axes through the first midpoint, which is situated (see FIG. 4) at a maximum of a first axial distance; that is at a maximum speed ratio, at LOW transmission ratio, corresponding with a minimum radial distance between the sheaves of the drive or primary pulley. The second output extends through a second midpoint, which is situated at a maximum of a second axial distance, that is at a minimum speed ratio, at a high transmission ratio, generally called OD (overdrive) corresponding with a minimum radial distance between the sheaves of the driven or secondary pulley. Both midpoints are derived from conditions wherein the transmission belt is subjected to a tension originating from a bending at its smallest radii of curvature at the inner sides of the drive and driven pulleys respectively. It is according to the prior art apparently assumed that under those conditions speed differences and slide movements between transverse elements and bands or rings holding these elements together to form the transmission belt, tend to be maximal. Accordingly it is proposed to direct the outputs of the lubrication nozzle at the geometrically derived midpoints of the transmission belt which midpoints are derived from two extreme outer and opposing transmission ratios.
It is the object of the present invention to provide an optimised manner in which the belt may be applied in a transmission and promote optimal cooling features of the belt and transmission.
Thereto the continuously variable transmission according to the invention is characterised in that at least one output of the lubrication nozzle at the driven pulley is directed to the midpoint of the transmission belt while running in a transmission ratio TOP area A, wherein a maximum power is transmitted by the transmission belt.
Accordingly the lubrication device for application in the continuously variable transmission and a vehicle provided with such a transmission have corresponding characterising features.
According to the invention the performance of the transmission has now been improved further, because use is made of the notion that if the transmitted power through the transmission and belt is maximal the power loss and associated generated heat of the variable transmission process is maximal tooxe2x80x94which is the case at the so called TOP transmission ratioxe2x80x94and cooling is theoretically and practically to be essential. According to an insight of the present invention, in said TOP ratio relatively more power gets lost, so that the absolute amount of power loss, meaning heat dissipation, through a certain percentage of slip occurs in that ratio. In particular at the secondary or driven pulley, where curvature of the belt elements between the secondary sheaves is strongest and the need for lubrication is correspondingly high this lubrication is now advantageously provided for. The relative larger amount of lost power is absorbed effectively if the output of the lubrication nozzle is directed such that account is taken of the actual secondary radial position of the belt, wherein this larger power is generated. Advantageously the so called splash temperature of lubricating fluid splashing from the rotating transmission belt is measured to be lowered. In an example the power loss increase between an OD (overdrive) transmission ratio and the TOP transmission ratio amounts a substantial 45%, here corresponding with 1.1 KW.
An effective cooling under such a maximum heat producing condition reduces the chances of overheating essential parts of the transmission constituents and of overheating the lubricating fluid, such as oil applied therein. The latter reduces the chances of oil disintegrating as a consequence of a possible exceeding of an allowable peak temperature of the oil and increases its life time as well as the capability of maintaining its lubricating properties during a longer period of time. If such a peak temperature is now locally exceeded at all the duration of such a local exceeding is advantageously reduced. Because of the reduced overheating the life time of the transmission and in particular the transmission belt is increased, and the maintaining of consistent and constant wear and tear properties will be prolonged, which improves durability and reliability during a longer operational period of time of the CVT.
In addition it has been observed by the inventor that because of the lubrication proposed the most inner band of a band pack which holds the transmission belt and its transverse elements together shows a decreased decay of ring surface roughness, which prolongs the lifetime of the transmission and in particular its transmission belt.
An embodiment of the continuously variable transmission according to the invention is characterised in that one other output of the lubrication nozzle at the drive pulley is directed to the midpoint of the transmission belt, while running in a transmission ratio LOW area B.
Power loss at the transmission ratio LOW end of the drive pulley is substantial too and may advantageously be combined with reduction at the transmission ratio TOP end of the driven pulley in order to arrive at an even further improved two sided cooling of the continuously variable transmission according to the invention.
A further embodiment of continuously variable transmission according to the invention is characterised in that the lubrication fluid emanating from the nozzle has a flow direction which is in agreement with the rotation direction of the drive and/or driven pulleys.
Advantageously no extra heat is generated because oil particle collisions at excessive velocity differences are avoided.
A still further embodiment of continuously variable transmission according to the invention is characterised in that the output of the lubrication nozzle is single or double respectively. In particular a double lubrication nozzle improves lubrication and cooling.
In another embodiment of the invented transmission, controllable amounts of the respective lubricating fluid yields through outputs O1 and O2 per unit time lie in the range between 1:2 and 2:1. This can be transmission ratio dependent.
In still another embodiment the lubricating fluid is supplied in one of more laminar flows, which reduces the lubrication fluid pump capacity, and the presence of air bubbles in the fluid, such as oil.
Another further embodiment of the continuously variable transmission according to the invention saving valuable space in between the pulleys, is characterised in that the lubrication nozzle comprises a single pipe entering the region between the drive and driven pulley. Preferably the single pipe is joined to a branch pipe forming the double output of the lubrication pipe, which saves valuable space between the pulley sheaves of the transmission according to the invention.